The singer died on Monday in Los Angeles, the family told the Associated Press.
They described him in a statement as a "solitary man with a heart driven to connect to the world".
"He spoke honestly to people and connected them to each other," the statement said.
Known for his smooth baritone vocals and sumptuous soul arrangements, he wrote some of the 70s best-remembered songs, including Just The Two Of Us, Lovely Day and Use Me.
On Lovely Day, he set the record for the longest sustained note on a US chart hit, holding a high E for 18 seconds.